MSC3266: Room summary API

proposals/3266-room-summary.md

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+# MSC3266: Room Summary API
+
+Quite a few clients and tools have a need preview a room:
+
+- A client may want to show the room in the roomlist, when showing a space.
+- matrix.to may want to show avatar and name of a room.
+- Nextcloud may want to list the names and avatars of your `/joined_rooms` when
+  asking where to share the media.
+- A client may want to preview a room, when hovering a room alias, id or after
+  clicking on it.
+- A client may want to preview a room, when the user is trying to knock on it or
+  to show pending knocks.
+- A traveller bot may use that to show a room summary on demand without actually
+  keeping the whole room state around and having to subscribe to /sync (or
+  using the appservice API).
+- A client can use this to knock on a room instead of joining it when the user
+    tries to join my room alias or link.
+
+There are a few ways to request a room summary, but they only support some of
+the use cases. The [spaces hierarchy API](https://spec.matrix.org/v1.3/client-server-api/#get_matrixclientv1roomsroomidhierarchy) only provides
+limited control over what rooms to summarize and returns a lot more data than
+necessary.  `{roomid}/initialSync` and `{roomid}/state/{event_type}` don't work
+over federation and are much heavier than necessary or need a lot of http calls
+for each room.
+
+## Proposal
+
+A new client-server API, which allows you to fetch a summary of a room by id or
+alias, and a corresponding server-server API to fetch a summary over federation.
+
+### Client-Server API
+
+The API returns the summary of the specified room, if the room could be found
+and the client should be able to view its contents according to the same rules
+[`/hierarchy`](https://spec.matrix.org/v1.3/client-server-api/#server-behaviour-19)
+endpoint. This is generally described as being a "potential joiner", which
+includes rules such as being a member of a room that allows joining a
+[`restricted`](https://spec.matrix.org/v1.3/client-server-api/#restricted-rooms),
+the room being
+[knockable](https://spec.matrix.org/v1.3/client-server-api/#knocking-on-rooms),
+public or you already being a member. For unauthenticated requests a response
+should only be returned if the room is publically accessible.
+
+A request could look like this:
+
+```
+GET /_matrix/client/v1/summary/{roomIdOrAlias}?
+    via=matrix.org&
+    via=neko.dev
+```
+
+(This is not under `/rooms`, because it can be used with an alias.
+
+- `roomIdOrAlias` can be the roomid or an alias to a room.
+- `via` are servers that should be tried to request a summary from, if it can't
+  be generated locally. These can be from a matrix URI, matrix.to link or a
+  `m.space.child` event for example.
+
+A response includes the stripped state in the following format:
+
+```json5
+{
+  room_id: "!ol19s:bleecker.street",
+  avatar_url: "mxc://bleecker.street/CHEDDARandBRIE",
+  guest_can_join: false,
+  name: "CHEESE",
+  num_joined_members: 37,
+  topic: "Tasty tasty cheese",
+  world_readable: true,
+  join_rule: "public",
+  room_type: "m.space",
+  membership: "invite",
+  encryption: "m.megolm.v100",
+  room_version: "9001",
+}
+```
+
+These are the same fields as those returned by `/publicRooms`, with a few
+additions: `room_type`, `membership`, `room_version` and `encryption`.
+
+`room_type`, `room_version` and `encryption` are already accessible as part of
+the stripped state according to
+https://spec.matrix.org/v1.3/client-server-api/#stripped-state , with the
+exception of `membership`. These are the same fields as in the response to
+[`/hierarchy`](https://spec.matrix.org/v1.3/client-server-api/#get_matrixclientv1roomsroomidhierarchy)
+apart from the addition of the `room_type`, `membership`, `room_version` and
+`encryption` fields. The membership can already be accessed by a client anyway,
+this API just provides it as a convenience.
+
+
+#### Rationale and description of response fields
+
+| fieldname          | description                                                                                                                                           | rationale                                                                                                                             |
+| ------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
+| room_id            | Required. Id of the room                                                                                                                               | Useful when the API is called with an alias or to disambiguate multiple responses clientside.                                        |
+| avatar_url         | Optional. Avatar of the room                                                                                                                          | Copied from `publicRooms`.                                                                                                            |
+| guest_can_join     | Required. If guests can join the room.                                                                                                                | Copied from `publicRooms`.                                                                                                            |
+| name               | Optional. Name of the room                                                                                                                            | Copied from `publicRooms`.                                                                                                            |
+| num_joined_members | Required. Member count of the room                                                                                                                    | Copied from `publicRooms`.                                                                                                            |
+| topic              | Optional. Topic of the room                                                                                                                           | Copied from `publicRooms`.                                                                                                            |
+| world_readable     | Required. If the room history can be read without joining.                                                                                            | Copied from `publicRooms`.                                                                                                            |
+| join_rule          | Optional. Join rules of the room                                                                                                                      | Copied from `publicRooms`.                                                                                                            |
+| room_type          | Optional. Type of the room, if any, i.e. `m.space`                                                                                                    | Used to distinguish rooms from spaces.                                                                                                |
+| room_version       | Optional (for historical reasons). Version of the room.                                                                                               | Can be used by clients to show incompatibilities with a room early.                                                                   |
+| membership         | Optional (1). The current membership of this user in the room. Usually `leave` if the room is fetched over federation.                                              | Useful to distinguish invites and knocks from joined rooms.                                                                           |
+| encryption         | Optional. If the room is encrypted this specified the algorithm used for this room. This is already accessible as stripped state. | Some users may only want to join encrypted rooms or clients may want to filter out encrypted rooms, if they don't support encryption or not this algorithm. |
+
+It should be possible to call this API without authentication, but servers may
+rate limit how often they fetch information over federation more heavily, if the
+user is unauthenticated.
+
+(1) The field `membership` will not be present when called unauthenticated, but
+is required when called authenticated. It should be `leave` if the server
+doesn't know the users membership state (for example if the server is not known
+to the local server).
+
+#### Modifications to `/_matrix/client/v1/rooms/{roomId}/hierarchy`
+
+For symmetry the `room_version` and `encryption` fields are also added to the
+`/hierarchy` API.
+
+### Server-Server API
+
+For the
+[`/hierarchy`](https://spec.matrix.org/v1.3/server-server-api/#get_matrixfederationv1hierarchyroomid)
+federation API is reused. This provides (with a few changes) all the information
+needed in this MSC, but it also provides a few additional fields and one level
+of children of this room.
+
+Additionally the `encryption` and `room_version` fields are added to the
+responses for each room.
+
+In theory one could also add the `max_depth` parameter with allowed values of 0
+and 1, so that child rooms are excluded, but this performance optimization does
+not seem necessary at this time and could be added at any later point while
+degrading gracefully.
+
+(Originally there was a separate federation API for this, but it was decided
+by the author that lowering the duplication on the federation side is the way to go.)
+
+## Potential issues
+
+### Performance
+
+Clients may start calling this API very often instead of using the
+[`/hierarchy`](https://spec.matrix.org/v1.3/client-server-api/#get_matrixclientv1roomsroomidhierarchy)
+for spaces or caching the state received via `/sync`.
+Looking up all the state events required for this API may cause performance
+issues in that case.
+
+To mitigate that, servers are recommended to cache the response for this API and
+apply rate limiting if necessary.
+
+## Alternatives
+
+- The 
+    [`/hierarchy`](https://spec.matrix.org/v1.3/client-server-api/#get_matrixclientv1roomsroomidhierarchy)
+    API could be used, but it returns more data than necessary
+    by default (but it can be limited to just 1 room) such as all the
+    `m.space.child` events in a space, but also is missing the room version,
+    membership and the encryption field. (We do reuse the federation API now.)
+- For joined rooms, the `/sync` API can be used to get a summary for all joined
+    rooms. Apart from not working for unjoined rooms, like knocks, invites and
+    space children, `/sync` is very heavy for the server and the client needs to
+    cobble together information from the `state`, `timeline` and
+    [`summary`](https://github.com/matrix-org/matrix-doc/issues/688) sections to
+    calculate the room name, topic and other fields provided in this MSC.
+    Furthermore, the membership counts in the summary field are only included, if
+    the client is using lazy loading.
+    This MSC provides similar information as calling `/sync`, but it uses the
+    stripped state, which is needed to allow this to work for unjoined rooms and
+    it excludes `m.heroes` as well as membership events, since those are not
+    included in the stripped state of a room. (A client can call
+    `/joined_members` to receive those if needed. It may still make sense to
+    include heroes so that clients could construct a human-friendly room display
+    name in case both the name and the canonical alias are absent; but solving
+    the security implications with that may better be left to a separate MSC.
+- The `/state` API could be used, but the response is much bigger than needed,
+    can't be cached as easily and may need more requests. This also doesn't work
+    over federation (yet). The variant of this API, which returns the full state
+    of a room, also does not return stripped events, which prevents it from
+    being used by non-members. The event for specific events DOES return
+    stripped events, but could not provide a member count for a room.
+- Peeking could solve this too, but with additional overhead and
+    [MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753) is much more
+    complex. You need to add a peek and remember to remove it. For many usecases
+    you just want to do one request to get info about a room, no history and no
+    updates. This MSC solves that by reusing the existing hierarchy APIs,
+    returns a lightweight response and provides a convenient API instead.
+- This API could take a list of rooms with included `via`s for each room instead
+    of a single room (as a POST request). This may have performance benefits for
+    the federation API and a client could then easily request a summary of all
+    joined rooms. It could still request the summary of a single room by just
+    including only a single room in the POST or a convenience GET could be
+    provided by the server (that looks like this proposal).
+- [MSC3429](https://github.com/matrix-org/matrix-doc/pull/3429) is an
+    alternative implementation, but it chooses a different layout. While this
+    layout might make sense in the future, it is inconsistent with the APIs
+    already in use, harder to use for clients (iterate array over directly
+    including the interesting fields) and can't reuse the federation API. In my
+    opinion an MSC in the future, that bases all summary APIs on a list of
+    stripped events seems like the more reasonable approach to me and would make
+    the APIs more extensible.
+
+## Security considerations
+
+This API may leak data, if implemented incorrectly or malicious servers could
+return wrong results for a summary.
+
+Those are the same concerns as on [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946)
+or [MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173).
+
+This API could also be used for denial of service type attacks. Appropriate
+ratelimiting and caching should be able to mitigate that.
+
+## Unstable prefix
+
+This uses the `im.nheko.summary` unstable prefix. As such the paths are prefixed
+with `unstable/im.nheko.summary`.
+
+- the client API will be
+    `/_matrix/client/unstable/im.nheko.summary/summary/{roomIdOrAlias}`.
+
+Some implementations still use
+    `/_matrix/client/unstable/im.nheko.summary/rooms/{roomIdOrAlias}/summary`,
+    but this was a mistake in this MSC. Endpoints using aliases shouldn't be under /rooms.
+
+Additionally the fields `encryption` and `room_version` in the summaries are
+prefixed with `im.nheko.summary` as well since it is new. The latter might still
+be called `im.nheko.summary.version` in some implementations.